Viewer: Viewers are in particular those who are watcing televison. A viewer could be also someone looking carefully at a picture or any other interesting object, like an artifact, sculptures in a museum, paintings and pieces of art in general. Spectator: It refers also to someone watching something ,that is more likely to be a public event or activity ,but specifically sport events such as tennis matches, horse racing events etc... Audience: Refers to the ones watching or listening to a rather publically held event such as concerts, plays, public meetings.The people come to a certain place to watch this event ,whether a movie, a performance of any kind or a speech. When you say 'the audience of a television or radio progamme' you refer to all the people as a group watching or listening to it. A writer or an artist could have 'an audience' and that would be the people who read his work or look at his work.

They can be used interchangeably quite often (the formal definition of audience actually calls it the viewers or spectators of an event), BUT Viewers and spectators are only to be used for those who are looking at something, whereas audience can mean the people who read a book or something like that - not everything they do is watch an event, unlike viewers/spectators. Ex. "The kids' book was written for a younger audience."